The high-precision measurement of a string's motion requires either the use of an expensive apparatus or the development of a dedicated system. In this paper, a cheaper alternative based on opto-switch sensors combined with a suitable calibration is proposed. A sensitivity model requiring only two straightforward preliminary measurements to determine the parameters is presented. A comparison on a bench test between the opto-switch sensor and a high-speed camera has been performed. Results indicate that the calibrated opto-switch provides more accurate measurements of the string's motion in quasi-static as well as in dynamic states.